(1) Field of the Invention
The present invention relates to thermally stable hybrid molecular sieve silicas generally having uniform pores, and specifically to calcined silicas. The silicas have hybrid wormhole and either lamellar or hexagonal structures intergrown together. In particular, the present invention relates to the use of water soluble silicates and preferably neutral amine surfactants surfactants for the preparation of these thermally stable silicas. In particular the present invention relates to mesoporous silicas having a pore size between about 1.0 and 12 nm.
(2) Description of Related Art
The disclosure by Mobil in 1992 (Beck, J. S., et al., J. Am. Chem. Soc. 114 10834 (1992)) of the synthesis of mesoporous aluminosilicate molecular sieves (M41S materials) utilizing assemblies of cationic organic molecules (micelles) as structure directors led to a vast amount of research into this field. To date, the synthesis of mesoporous molecular sieves can be classified into several general pathways according to their organic-inorganic interfacial interactions. Electrostatic charge matching (Beck, J. S., et al., J. Am. Chem. Soc. 114 10834 (1992); Huo, Q., et al., Chem. Mater. 6 1176 (1994); Huo, Q., et al., Nature 368 317 (1994)), H-bonding (Tanev, P. T., et al., Science 267 865 (1994); and Bagshaw, S. A., et al., Angwen. Chem. Int. Ed. Engl. 36 516 (1997)), and dative bonding interactions (Antonelli, D. M., et al., Angwen. Chem. Int. Ed. Engl., 35 426 (1996); and Antonelli, D. M., et al., Chem. Mater 8 874 (1996)) at the organic micelle-inorganic interface have all been successfully utilized in the formation of mesostructured inorganic materials.
Electrostatic charge matching pathways utilize coulombic interactions between the charged structure directing surfactant assemblies (micelles) and ionic silica species in the assembly of stable inorganic framework structures. As reported by Mobil, synthesis of the M41S family of molecular sieves relies on cooperative assembly between cationic quaternary ammonium surfactant micelles (S+) and anionic water-soluble silicates (Ixe2x88x92). Synthesis under hydrothermal conditions results in mesoporous silicates that possess a high degree of framework pore order. M41S materials are generally large particle materials that have uniform pore diameters, significantly large surface areas (xe2x88x92800-1200 m2/g) and little to no observable textural mesoporosity (Tanev, P. T., et al., Chem. Mater. 8 2068 (1996)). Due to the strong coulombic interactions between the surfactant and the silica wall, however, a simple solvent extraction and recycling of this costly quaternary ammonium surfactant is not possible. Surfactant removal is accomplished either by calcinations or by an ion exchange-solvent extraction method (Whitehurst, D. D. U.S. Pat. No. 6,143,879 (1992)).
The syntheses of HMS materials rely on H-bonding interactions between the neutral amine surfactant (So) assemblies and molecular silica precursors (Io) such as tetraethylorthosilica (TEOS) (Tanev, P. T., et al., Science 267 865 (1995)). This H-bonding interaction is significantly weaker than the coulombic interactions of the electrostatic pathways resulting in the disordered wormhole pore structure typical of HMS silicas (Tanev, P. T., et al., Science 267 865 (1995); Tanev, P. T., et al., Chem. Mater. 8 2068 (1996); and Behrens, P., Angew. Chem. Int. Ed. Engl. 35(5) 515 (1996)). This wormhole pore structure has significant pore branching and 3-dimensional pore character. Characteristic properties of HMS silicas, however, are similar to those of electrostatically assembled mesostructures in their pore size distributions, surface areas, and pore volumes. Additionally, synthesis of these silicas in highly polar solvents, where the surfactant exists in an emulsion phase, results in small particle materials that possess significant textural, or inter-particle, porosity (Pauly, T. R., et al., J. Am. Chem. Soc. 121 8835 (1999)). This fact along with the highly branched pore structure yields a mesoporous material that exhibits unique catalytic activity due to the enhanced access to reactive sites (Pauly, T. R., et al., J. Am. Chem. Soc. 121 8835 (1999)).
Long alkyl chain amine surfactants used in HMS synthesis are significantly less costly than quaternary ammonium salts used in the synthesis of M41S and SBA materials. The use of TEOS or other molecular silica species, however, is considerably more expensive than available water soluble silicate species. Thus far, however, mesostructure synthesis using H-bonding mechanisms with neutral amine surfactants required the use of molecular silica species.
Mesoporous molecular sieve silicas with wormhole framework structures (e.g., MSU-X (Bagshaw, S. A., et al., Science 269 1242 (1994); Bagshaw, S. A., et al., Angwen. Chem. Int. Ed. Engl. 35 1102 (1996); Prouzet, E. et al., Angwen. Chem. Int. Ed. Engl. 36 516 (1997), and HMS (Tanev, P. t., et al., Science 267 865 (1995)) are generally more active heterogeneous catalysts in comparison to their ordered hexagonal analogs (e.g., MCM-41 (Beck, J. S., et al., J. Am. Chem. Soc. 114 10834 (1992); and Huo, Q., et al., Nature 368 317 (1994)), and SBA-15 (Stucky, JACS). The enhanced reactivity has been attributed, in part, to a pore network that is connected in three dimensions, allowing the guest molecules to more readily access reactive centers that have been designed into the framework surfaces (Tanev, P. T., et al., Chem. Mater. 8 2068 (1996); Whitehurst, D. D. U.S. Pat. No. 6,143,879 (1992); Behrens, P. Angwen. Chem. Int. Ed. Engl. 35(5), 515 (1996); and Pauly, T. R., et al., J. Am. Chem. Soc. 121 8835 (1999)). All of the wormhole framework structures reported to date have been prepared through supramolecular SoIo (Tanev, P. T., et al., Science 267 865 (1995) and NoIo (Bagshaw, S. A., et al., Angwen. Chem. Int. Ed. Engl. 35 1102 (1996); Prouzet, E., et al., Angwen. Chem. Int. Ed. Engl. 36 516 (1997)) assembly pathways wherein Io is an electrically neutral silica precursor (typically, tetraethylorthosilicate, TEOS), So is a neutral amine surfactant, and No is a neutral di- or tri-block surfactant containing polar polyethylene oxide (PEO) segments. One disadvantage of these pathways, as with other assembly pathways based on TEOS, is the high cost of the hydrolyzable silicon alkoxide precursor. Greater use of wormhole framework structures as heterogeneous catalysts can be anticipated if a more efficient approach to either SoIo or NoIo assembly is devised based on the use of low cost soluble silicate precursors, without sacrificing the intrinsically desirable processing advantages of these pathways (e.g., facile removal and recycling of the surfactant).
Recently, Guth and co-workers reported the preparation of disordered silica mesostructures by precipitation from sodium silicate solutions over a broad range of pH in the presence of TRITON-X 100, an No surfactant (Sierra, L., et al., Adv. Mater 11(4) 307 (1999); and Sierra, L., et al., Microporous and Mesoporous Materials 27 243 (1999)). The retention of a mesostructure was observed up to a calcination temperature of 480xc2x0 C., but the complete removal of the surfactant at 600xc2x0 C. led either to the extensive restructuring of the silica framework, as indicated by the loss of mesoporosity or the formation of a completely amorphous material. In contrast wormhole MSU-X and HMS mesostructures are structurally stable to calcination temperatures in excess of 800xc2x0 C.
Of interest is the use of an aqueous acid solution to extract an amine surfactant template from the as-formed mesoporous silica composition. This is reported by Cassiers et al., Royal Society of Chemistry 2489-2490 (2000).
U.S. Pat. Nos. 5,800,799, 6,027,706, 5,622,684, 5,795,559, 5,855,864, 5,672,556, 5,840,264, 5,800,800, 5,785,946, and 5,712,402, are generally related to the present invention.
Objects
There is a need for mesoporous silica compositions with improved properties. There is also a need for mesostructured silica compositions which are economical to prepare. These and other objects will become increasingly apparent by reference to the following description and the drawings.
The present invention relates to hybrid mesoporous silica compositions in which the framework pore structure is defined by the intergrowth of nano-domains of both wormhole framework pore structures and either lamellar or hexagonal framework pore structures. Typically, the nano-domains are of 100 nm or less in diameter and do not possess a distinct boundary between adjacent domains.
The present invention relates to a hybrid molecular sieve silica composition comprising a framework structure defining the mesopores which is in one domain lamellar or hexagonal and in another domain with wormhole pores and wherein the domains are intergrown together. Typically the domain sizes are 100 nm or less.
The present invention also relates to a composition which is a hybrid wormhole and lamellar or hexagonal framework molecular sieve silica prepared by a neutralizing reaction in an aqueous solution of amine surfactant; a reactive silica species of pH balance 5 and 10.5; aging of the solution to precipitate the silica and removing of the silica from the solution.
The present invention particularly relates to a composition which is a hybrid wormhole and lamellar or hexagonal molecular sieve silica prepared by a process which comprises:
(a) acidifying an aqueous solution of an amine surfactant, preferably containing 6 to 36 carbon atoms, as a structure director with an acid selected from the group consisting of organic, mineral and oxy acids;
(b) preparing a reactive silica species in the aqueous solution by neutralization of a basic soluble silicate solution by mixing with the acidified amine surfactant aqueous solution of step (a) reaching a final pH of about 5 to 10.5;
(c) aging the reactive silica species from step (b), preferably for no less than 5 minutes, at a temperature greater than xe2x88x9220xc2x0 C. in anhydrous form. The silica has the formula:
(c) aging the reactive silica species from step (b), preferably for no less than 5 minutes, at a temperature greater than xe2x88x9220xc2x0 C. in anhydrous form. The silica has the formula:
SiMwO2+x
wherein 1.0xe2x89xa7wxe2x89xa70 and 1.5xe2x89xa7xxe2x89xa70 and wherein M when present is one or more metal ions.
wherein 1.0xe2x89xa7wxe2x89xa70 and 1.5xe2x89xa7xxe2x89xa70 and wherein M when present is one or more metal ions.
(d) recovering a solid product from the aqueous solution by removal of the solution; and
(e) removing the surfactant from the solid by calcination at 600xc2x0 C. in air for not less than 30 minutes, by solvent extraction, or by treatment with a stoichiometric amount of aqueous acid solution and washing with water, to produce the molecular sieve silica, wherein silica possesses framework-confined mesopores with pore diameters ranging from 1.0 to 12.0 nm, the framework-confined channel structure comprises a hybrid wormhole and lamellar or hexagonal framework morphology has at least one resolved powder x-ray reflection corresponding to a pore-pore correlation spacing of 1.5 to 15.0 nm, inorganic oxide wall thickness of greater than 0.5 nm, specific surface areas of 400 to 1400 m2/g and framework pore volumes of 0.2 to 2.0 cc/g N2, preferably with textural pore volumes of 0.01 to 3 cc/g N2.
The present invention further relates to a composition which is a hybrid molecular sieve silica prepared by a process that comprises:
(a) preparing an aqueous solution of a amine surfactant as an organic structure director;
(b) adding a basic soluble silicate to the amine solution;
(c) neutralizing the basic amine and silicate solution with an acid selected from the group consisting of organic, mineral and oxy acids to a final pH of about 5.0 to 10.5 to provide a reactive silica;
(d) aging reactive silica from step (b) at temperatures greater than xe2x88x9220xc2x0 C.;
(e) recovering a solid product from the aqueous solution; and
(f) removing the surfactant by removal of the solution to provide the molecular sieve silica, wherein the silica possesses framework-confined mesopores with pore diameters ranging from 1.0 to 12.0 nm, the framework-confined channel structure comprises the hybrid, a wormhole and lamellar or wormhole framework morphology, has one resolved powder X-ray reflection corresponding to a pore-pore correlation spacing of 1.5 to 15.0 nm, inorganic oxide wall thickness of greater than 0.5 nm, specific surface areas of 400 to 1400 m2/g and framework pore volumes of 0.2 to 3.0 cc/g N2, and preferably textural pore volumes of 0.01 to 3 cc/g N2.
The present invention further relates to a composition which is a hybrid molecular sieve silica prepared by a process which comprises:
(a) acidifying an aqueous solution of an amine surfactant containing an alkyl chain with 6 to 36 carbon atoms as the organic structure director with an acid selected from the group consisting of organic, mineral and oxy acids;
(b) preparing a reactive silica species by addition of a soluble silicate to the acidified amine surfactant reaching a pH of less than 4;
(c) titrating the reactive silica with a base to a final pH of about 5.0 to 10.5;
(d) aging reactive silica from step (b) at temperatures greater than xe2x88x9220xc2x0 C.;
(e) recovering a solid product from the aqueous solution; and
(f) removing the surfactant from the solid product to provide the molecular sieve silica, wherein the resulting inorganic oxide possesses framework-confined mesopores with pore diameters ranging from 10 to 12.0 nm, the framework-confined channel structure comprises the hybrid of a wormhole and lamellar or hexagonal framework morphology, has at least one resolved powder x-ray reflection corresponding to a pore-pore correlation spacing of 1.5 to 15.0 nm, inorganic oxide wall thickness of greater than 0.5 nm, specific surface areas of 400 to 1400 m2/g and framework pore volumes of 0.2 to 2.0 cc/g N2, and textural pore volumes of 0.01 to 3 cc/g N2.
The present invention relates to a process for the preparation of a hybrid wormhole and lamellar or hexagonal molecular sieve silica which comprises:
(a) reacting in an aqueous solution, an amine surfactant and a reactive silica species of pH between 5 and 10.5;
(b) aging the solution to precipitate the silica; and
(c) removing the silica from the solution.
The present invention further relates to a process for the preparation of a hybrid molecular sieve silica which comprises:
(a) providing a protonated amine surfactant solution with a pH below 7.0;
(b) reacting the protonated amine surfactant solution with a mixture of a base and a soluble silicate solution to produce a reactive silica species at a final pH between about 5 and 10.5;
(c) aging the reactive silica species in the solution of step (b) at a temperature greater than xe2x88x9220xc2x0 C. to form a precipitated product which is the silica in the solution; and
(d) recovering the precipitated product from the solution.
The present invention further relates to a process for the preparation of a hybrid molecular sieve silica which comprises:
(a) acidifying a surfactant solution of a neutral amine surfactant with an acid thereof to produce a pH below 7.0;
(b) forming a reactive silica species by neutralization of a soluble silicate solution with the surfactant solution of step (a) to provide a final pH of about 5.0 to 10.5;
(c) aging the reactive silica species in the solution of step (b) at a temperature greater than xe2x88x9220xc2x0 C. to form a precipitated product which is the silica in the solution; and
(d) recovering the precipitated product from the solution.
The present invention further relates to a process for the preparation of a hybrid molecular sieve silica which comprises:
(a) providing an aqueous solution of a water soluble silicate at a pH greater than 9;
(b) combining the aqueous solution with a neutral amine surfactant and an acid to produce a resulting mixture wherein the pH of the mixture is between about 5.0 and 10.5;
(c) aging the resulting mixture at a temperature between xe2x88x9220xc2x0 and 100xc2x0 C. until the hybrid molecular sieve silica is formed; and
(d) removing at least the aqueous solution to produce the hybrid molecular sieve silica.
The present invention further relates to a process for the preparation of a hybrid molecular sieve aluminosilicate which comprises:
(a) providing an aqueous solution of a water soluble aluminate and silicate in a molar ratio of aluminate to silicate of between about 0.01 and 1.0 at a pH greater than 9;
(b) combining the aqueous solution with neutral amine surfactant and an acid in aqueous solution to produce a resulting mixture wherein the pH of the mixture to be between about 5.0 and 10.5;
(c) aging the resulting mixture at a temperature between xe2x88x9220xc2x0 and 100xc2x0 C. until the hybrid molecular sieve aluminosilicate is formed; and
(d) removing at least the aqueous solution to produce the hybrid molecular sieve aluminosilicate.
Further the present invention relates to a process for the preparation of a hybrid molecular sieve alumino-silicate which comprises:
(a) providing an aqueous solution of a water soluble silicate at a pH greater than 9;
(b) combining the aqueous solution with a neutral amine surfactant, an aluminum salt and an acid in aqueous solution to produce a resulting mixture wherein the aluminum to silicon molar ratio is between 0.01 and 1.0 and the pH of the mixture to be between about 5.0 and 10.5;
(c) aging the resulting mixture at a temperature between xe2x88x9220xc2x0 and 100xc2x0 C. until the hybrid molecular sieve aluminosilicate is formed; and
(d) removing at least the aqueous solution to produce the hybrid molecular sieve aluminosilicate.